1. Field of the Invention
The present invention generally relates to light emitting devices using light emitting diodes, and more particularly to a light emitting device having a rectifier circuit integrated in the substrate and a related fabrication method thereof.
2. The Prior Arts
In general, an appropriate forward-biased DC voltage is applied to a light emitting diode (LED), and photons are thereby released as the electrons and holes of the LED are driven and re-combined with each other. Recently, as the applications of LEDs are extended to home and outdoor lighting applications, there are increasing demands for directly driving LEDs using AC voltages. However, if an AC voltage is applied on a LED, the LED would be damaged if an excessive reverse-biased voltage is exerted across the LED. A common solution is to adopt a transformer or a voltage reduction circuit or the like to reduce the level of the AC voltage so that the LED can withstand. The solution is effective but the price to pay is that a significant amount of electrical energy is converted to heat and wasted in the process.
Another problem associated with driving LEDs with an AC voltage is that the LEDs would be lit only during the half cycle that the LEDs are forward-biased. During the other half cycle, the LEDs are turned off as they are reversed-biased. As such, driving LEDs with an AC voltage is usually integrated with a rectifier circuit to turn the AC voltage into a DC voltage before it is applied on the LEDs. The most common rectifier circuit is the bridge rectifier circuit.
FIG. 1a is a schematic diagram showing a conventional approach in applying an AC voltage to a LED. As illustrated, the AC voltage from an AC power source (e.g., the 110V or 220V mains) 10 first passes through a voltage reduction device 20 (e.g., a resistor), and then rectified by the diodes 30 at the four arms of the bridge rectifier circuit, and then applied to the load 40 of the bridge rectifier circuit. FIGS. 1b and 1c are schematic diagrams showing the current path (in dashed lines) during the first half and second half of the AC voltage's each cycle. As illustrated, due to the function of the bridge rectifier circuit, the LED 40 (i.e., the load to the bridge rectifier circuit) is always forward-biased and lit during both half cycles.
Based on the same principle, as shown in FIG. 1d, a light emitting device disclosed by Republic of China, Taiwan Patent No. 265,741 arranges additional LEDs 40 in series connection along the four arms of the bridge rectifier circuit so that they will be lit at appropriate half cycles of the AC voltage as well.
In implementing the foregoing light emitting devices, conventionally, the appropriate circuit is first formed on a printing circuit board (PCB) or a substrate 50. Then, the diode dies 60 constituting the bridge rectifier circuit and the LED dies 70 are individually turned upside down so that their electrodes face downward towards the substrate 50, as illustrated in FIG. 1e. Finally, solder balls or metallic bumps 80 are used to connect the electrodes of the dies 60 and 70 to the appropriate location of the circuit on the substrate 60. Another conventional approach is to form the diodes of the bridge rectifier circuit and LEDs together on a substrate. Then, these diodes and LEDs are wire-bonded into the desired circuit using gold wires. The two approaches are all too complicated, contributing to rather lengthy production time and high production cost.